1. Field of the Invention
Embodiments of the invention relate to formation of nanofeatures in substrates and more particularly relate to chemically formed nanofeatures having dimensions smaller than previously available.
2. Description of the Related Art
DNA is used extensively in bottom-up nanofabrication to produce discrete or periodic nanostructures with controlled morphology. Such nanoscale DNA patterns have been utilized as templates for the deposition of nanoparticles, metals, semiconductors, carbon nanotubes, proteins, etc. The properties of DNA-templated constructs are directly and sometimes adversely influenced by the nucleic acid within the assembly. For example, electroless deposition of metal on DNA typically yields granular, polycrystalline nanowires because the mesoscale uniformity of the DNA molecule promotes simultaneous, multiplexed nucleation.
One such patterning approach was reported by Deng and Mao, (see Angew. Chem. 2004, 116, 4160-4162; Angew. Chem. Int. Ed. 2004, 43, 4068-4070.), who deposited thick metal films over DNA nanostructures on mica to create imprinted metal replicas of the DNA features with nanometer resolution.
Shadow or angled deposition is a well-known microfabrication technique used to produce features smaller than those directly achievable by optical lithography. Typically, a material is either deposited through a shadow mask or onto a textured surface, which serves as its own shadow mask for certain deposition directions. Common textured surfaces used for shadow deposition include self-organized crystalline facets of NaCl, sapphire, semiconductors, and substrates with microfabricated features. More recently, nanoparticles, nanowires, and carbon nanotubes have also been utilized in shadow deposition to enhance nanofabrication resolution, and patterns with 50 nm dimensions or smaller can be made by shadow deposition techniques. (See Jiang, P., McFarland, M. J., J. Am. Chem. Soc. 2005, 127, 3710-3711; Shumaker-Parry, J. S., Rochholz, H., Kreiter, M., Adv. Mater. 2005, 17, 2131-2134; Lu, Y., Liu, G. L., Kim, J., Mejia, Y. X., Lee, L. P., Nano Lett. 2005, 5, 119-124; Ancona, M. G., Kooi, S. E., Kruppa, W., Snow, A. W., Foos, E. E., Whitman, L. J., Park, D., Shirey, L., Nano Lett. 2003, 3, 135-138; Yan, X.-M., Kwon, S., Contreras, A. M., Bokor, J., Somorjai, G. A., Nano Lett. 2005, 5, 745-748; De Poortere, E. P., Stormer, H. L., Huang, L. M., Wind, S. J., O'Brien, S., Huang, M., Hone, J., Appl. Phys. Lett. 2006, 88, 143124.)
Smaller dimensions have been achieved through non-chemical methods such as electron beam lithography that utilize expensive instruments and have a variety of drawbacks. For example, processing a substrate by electron beam lithography to form nanoscale structures can require a day or more of instrument operation. Many flaws can be introduced such as flaws in the large amounts of data being processed during the lengthy instrument operation. Also, keeping the substrate physically stable during instrument operation is more difficult for the long processing times. Furthermore, electron beam lithography causes forward and back scattering of primary and secondary electrons that can adversely affect the quality of the features being placed in the substrate. Repeatability and control at the resolution limits are poor for electron beam lithography. Because of electron scatter and the travel of primary electrons through the material of the substrate, features generally must be spaced apart to avoid “so-called” proximity effects with electron beam lithography. Nested features cause lower quality in the adjacent structures due to exposure of other features to electrons forming adjacent structures. Thus, electron beam lithographyhas limitations in placing a plurality of closely spaced features on a substrate and in producing walls that have not been damaged by electrons.
Another method for forming nanoscale structures is scanning probe lithography, which utilizes a stylus for contacting the substrate in either a constructive or a destructive mode. Material in the form of chemical species can be added to the substrate via the stylus for constructive modification. Alternatively in the destructive modes, the substrate is manipulated by energy imparted to the substrate through the stylus. The energy can be mechanical, thermal, photonic, ionic, electronic, or X-rays. Scanning probe lithography has drawbacks including the need to frequently change the stylus, and the potential for damage through physical contact of the stylus with the substrate. Also, the scanning probe lithography methods have inferior resolution than can be achieved by electron beam lithography.